Loud speaker



Aug. 18, 1931.

A. A. THOMAS 1,819,210

LOUD SPEAKER Filed Aug. 21, 1928 INVENTOR Patented Aug. 18, 1931 PATENT OFFICE UNITED STATES ADOLPH A. THOMAS, OF NEW YORK, .N. Y., ASSIGNOR TO UNITED REPBOI JUGERS GQRBQBATION, A GORPORAT'ION OF NEW JERSEY LOUD SPEAKER Application filed August 21, 1928. Serial No. 300,994.

This invention relates to loudspeakers of the so-called dynamic type in which a coil vibrates in a magnetic field. The instrument of my invention embodies various fea- Ia' tures of novelty to provide a Construction having certain practical advantages over prior devices of this kind.

The most common form of dynamic loudspeaker on the market has a cylindrical 1% speech coil mounted on the back of a cone diaphragm and arranged to vibrate in an annular magnetic airgap. For this purpose it is necessary to mount the speech coil over the same pole piece that carries the 'magnetizing coil. As a result of this close magnetic coupling between the two coils,.the variable current impulses in the speech coil induce a disturbing transformer effect which tends to distort the reproduction. Manufacturers have endeavored to get around this difiiculty by adding a compensating coil, but that complicates the instrument and adds to its cost.

One of the purposes of the present invention is to overcome the foregoing objection by providing a magnet structure having two narrow airgaps in which two sides of a fiat speech coil are arranged to vibrate. The axis of the coil is atright angles to the axis of the magnetizing coil or coils, and is furthermore midway of the magnetic pole pieces, so that the inductive efiect of the flat speech coil on the field flux is practically zero or at least is reduced to a negligible minimum. In a preferred embodiment of my invention, the speech coil consists of a fiat conducting ribbon wound in a single plane on a narrow frame of light non-'Inagnetic material. This ribbon may be transversely corrugated to provide air spaces for dissipating the heat. The coil frame is suspended at opposite ends between a pair of tensioned spring members which permit bodily vibration of the frame, and therefore of the coil, in its own plane. A loud-speaking diaphragm is attached to the coil frame and may be entirely supported thereby, so that a small freeedged diaphragm can be-used for repro'duclng with adequate volume. The

tension of the spring members that "support the coil may :be adjustable to produce the most efficient operation of the instrument in any particular case. Thefield magnet and coil frame are mounted'on the same support, and the-construction as a whole is characterized by simplicity and compactness.

The foregoing and other novel features and advantages of my invention will beiully understood from a detailed ldescriptionof the accompanying drawings in,wl1ich- Fig. 1 represents a loudspeaker constructed in accordance with my invention, this view being a section on line .l1 of 2;

Fig. :2 is a sectional view at right angles to Fig. 1 on line 2 2;

Fig. 3 shows a plan view partlyin section on line 33 of Fig. 1; r

Fig. Al is .a detached prespective view of the coil-supporting frame;

Fig. 5 is a fragmentary detail of the frame, partly insectiomon an enlarged :scale; and

Fig. 6 shows a coil in the form of :a transversely corrugated ribbon mounted on a slotted frame. v

The various parts of the instrument are mounted on a suitable support indicated :as a whole by 10, which may conveniently be cast as a single piece of suitable non-mag netic material, suchas aluminum, ba'kelite and others. In the present instance, the support 1 0 is a flat rectangular structure comprising two end pieces 12 connected by two pairs of'rods or posts 13 and 14. It is evident that each pair of posts may be a single plate. In other words, theposts 13 and limay be considered as plates with the central portions cut away to removeexce'ss weight. If desired, the posts 13 and 14 may be separately attached to the end pieces 12 by screws 15, or otherwise.

The field magnet structure comprises a pair of electromagnets M andM", each of which consists of a pair of pole pieces 16 connected by a magnetic core 17. A magnetizing'coil 18 is mounted on each core between the pole pieces 16, which are rigidly secured to the cores by 'bolts 19, preferably of magnetic metal. The pole pieces 16 are attached to the posts 13 and 14 loyfastening members 30, which may be bolts, screws, rivets zand the 15C like. The parts 16 and 17 of the magnetic field frames are preferably of material having a high magnetic permeability and of sufficient area to provide a powerful magnetic field without saturation. The coils 18 are connected in series or in parallel in a circuit of substantially constant current to furnish a magnetic field of the requisite strength. As seen in Fig. 3, the two pairs of pole pieces 16 provide two narrow airgaps 21 in alignment, and the field flux passes in series across these airgaps. The heads 16 of the pole pieces 16 are enlarged to increase the depth of the airgaps for a purpose that will presently become clear. By making the pole pieces 16 separate from the core 17 it is possible to use machine-wound coils and slip them over the cores before the pole pieces are attached. However, the magnet frames M and M may each be cast as a single piece of iron or magnetic alloy, and these frames may be solid or laminated.

f The vibratory coil of this instrument is wound on a flat rectangular frame indicated as a whole by 22 and best shown in Figs. 4 and 5. This frame is preferably of light nonmagnetic sheet metal like aluminum or duralumin, but it may also be made of thin sheet brass or other non-metallic material sufiiciently strong and not too heavy. A simple way to construct the frame 22 is to make it in two complementary parts adapt ed to be telescoped and fastened at the base by rivets 23, as best shown in Fig. 5. The

assembled frame comprisesa rectangular base section 24 and a pair of flanges 25 which define a narrow space 26 adapted to receive a coil 27. This coil may be of ordinary copper wire, but I prefer to use a thin flexible ribbon of non-magnetic conducting metal like aluminum, duralumin, copper, brass and the like. The width of this ribbon is slightly less than the width of the space 26, so that the turns of the coil arewound over each other in a single plane between the flanges 25 of frame 22, as shown in Figs. 1, 2 and 3. It goes without saying that the turns of the coil are insulated from each other in any practical way. An insulating tape may bewound with the coil ribbon between the turns thereof, or the ribbon itself may have a coating of suitable shellac to insulate the individual turns'of the coil not only from each other but also from the metal of the frame 22. The inner end 28 of coil 27 extends through an opening in the base of frame 22 to permit the attaching of a circuit lead 29. The other circuit conductor'3O is connected to the outer end 31 of the coil. It is understood that the coil 27 is connected by the conductors 29 and 30 to the output circuit of a suitable amplifying system, as will be clear to those who are skilled in this particular art. To hold the coil 27 in firmly wound condition on the rectangular supporting frame 22,1 provide the flanges 25 with integral lugs 32, which are bent into contact with the outer turn of the coil after the same has been wound on the frame. In other words, the lugs 32 are stamped out as integral extensions of the flanges 25, so as not to interfere with the winding of the coil on the frame either by hand or by machinery. After the requisite length of coil ribbon has been wound on the frame, it is a simple matter to bend the lugs 32 into firm pressure contact with the outer turn of the coil.

The coil frame is provided at opposite ends with integral. lugs 33 which terminate in lateral extensions 34. A spring blade 35 is connected to each pair of lateral extensions 34 by rivets 36 or other securing means. The lateral extensions 34 have holes 37 for receiving the fastening members 36. The outer ends of the spring blades 35 are attached to blocks 38 by rivets 39 or in any other practical way. The blocks 38 may be rigid, but I prefer to have them adjustably mounted on the support 10 by means of screws 40 which pass through holes in the end pieces 12 into screw-threaded engagement with the blocks. It is evident that, by adjusting one or both screws 40, the normal tension of springs 35 is regulated. To prevent turning or twisting of the spring blades 35 during adjustment of screws 40, the blocks 38 may be square and arranged to slide in square recesses 41in the end pieces 12 of frame 10. It will be observed in Fig. 1 that the tensioned suspension springs are arranged in line substantially midway of the coil frame 22 and in a plane at right angles to the plane of movement of the frame. The coil 27 is therefore free to move bodily in its own plane in the aligned air-gaps 21', and the flat springs 35 prevent lateral displacement of the coil frame so that the flanges 25 cannot strike. the adjacent faces of pole pieces 16. This permits the use of a frame which is only very slightly narrower than the magnetic airgaps 21, in which the two vertical sides 27 of coil 27 operate. In the broader aspect of my invention, the elastic suspension members 35 may be of other practical form, such as coil springs, taut wires, and the like. To increase the elasticity of the spring blades 35, they may be transversely corrugated.

- The coil frame 22 has a pair of integral extensions 42 to which a short rod 43 or other connecting member is rigidly attached by rivets 44. The connecting rod 12s a screwthreaded axial opening 45 for receiving a screw 46 adapted to secure a loud-speaking diaphragm 47 to the rod. The diaphragm 47 is rigidly clamped in position between a pair of conical disks 48 and 49. The disk 48 may be a flared extension formed integral with one end of rod 43. The diaphragm 47 may be free at the edge, since it is rigidly supported by the vibratory coil frame 22. There is,

lUu

however, no objection to providing a flexible support at the periphery of the diaphragm, which is of suitable sheet material such as aluminum, duralumin, paper, stiffened fabric, celluloid and the like. The diaphragm may be of plain conical shape, or it may be wholly or partly corrugated, as indicated at 50 in Fig. 1. In the completed instrument as made for the market, the diaphragm 4? is surrounded by a casing which encloses the main support 10 and the parts carried thereby, leaving the front side of the diaphragm open to the outer air. This will be understood without the need of additional illustration.

The operation of the instrument will be understood without detailed explanation. It is sufiicient to say that, as current impulses pass through the coil, the sides 2'? thereof move across the airgaps 21 in a direction at ight angles to the magnetic lines of force. The coil frame therefore vibrates bodily in its own plane, the extent and direction of this vibration depending on the intensity and direction of the current flowing through the coil. The tensioned springs 35 offer the right amount of load or mechanical impedance to the movements of the coil frame and always tend to restore the same to normal position. By making the frame 22 and coil 27 of light material (e. aluminum, duralumin, etc). the vibratory unit has minimum inertia and is highly sensitive to current variations, so that the entire musical scale is reproduced with substantially uniform efficiency.

In the modification of Fig. 6, the coil ribbon 27 is formed with transverse corrugations 270, which provide air spaces between the turns of the coil. These air spaces serve to dissipate the heat to which the coil is subjected by the passage of the electric current. The air spaces provided by the corrugations 27a prevent over-heating of the coil, even under heavy load. To facilitate still further the dissipation of heat, the flanges 25 of the coil frame may be slotted to provide ventilating openings or passages 51, so that the turns of the coil are freely exposed to the surrounding air. The casing in which the parts are housed is preferably provided with open in gs through which the air may circulate to carry away the heat generated inside the easing during the operation of the instrument. By making the speech coil 27 of ribbon instead of wire, it is capable of carrying a larger amount of current without over-heating, so that the instrument will operate efficiently even under an unusual increase of load. When in certain claims I refer to the coil and its supporting frame as being of aluminum, I do not mean only pure aluminum but any alloy of aluminum suitable for the purpose.

Although I have shown and described a specific construction, I want it understood that my invention is not limited to the details set forth. It is to be expected that changes and modifications will probably suggest themselves to the skilled worker in this art within the scope of the invention as defined in the appended claims.

I claim as my invention:

1. In an electric instrument, a vibratory coil consisting of an insulated ribbon wound in a'single plane and having transverse corrugation-s to provide air spaces.

In an -:electricinstriiment, a vibratory unit comprising a fiat frame having substantially p rallel flanges with cutaway portions, and a coil consisting of an insulated ribbon wound between said flanges and having transverse corrugations to provide air spaces.

in an electrodynamic loudspeaker, a vibratory unit comprising .a fiat frame of nonmagnetic material having substantially par allel flanges, an insulated conducting ribbon wound on said frame betweensaid flanges in a single plane, andmetallic means on said frame en aging the outer turn of ,said ribbon to hold the same in wound condition on the frame. 7

l. In an acoustic device, a vibratory coil support consistin of a flat and substantially rectangular frame of non-magnetic sheet metal, said frame comprising a pair of flanged sections secured together in telescoped relation, 5. In an acoustic device, a vibratory coil support consisting of a flat and substantially rectangular frame of non-magnetic sheet metal, said frame comprising a pair of flanged sections secured together in telescoped relation, the flanges of said sections being provided with integral lugs, and means connectedto said lugs for operatively support: ing the frame.

6. In an electrodynamic loudspeaker, a vibratory unit comprising a flat frame of nonmagnetic material having substantially arallel flanges, an insulated conducting rib on Wound on said frame between said flanges in a single plane, and one or more integral lugs projecting from the edge of said frame and bent laterally over the outer turn of said ribbon to hold the same in wound condition on the frame.

7. In an acoustic device, a vibratory coil support consisting of a flat and substantially rectangular frame of non-magnetic sheet metal, said frame comprising a pair of flanged sections secured together in telescoped relation, integral extensions on the flanges of said sections for connecting a diaphragm to said frame, and means for supporting said frame for vibratory movement.

8. In an electrodynamic loudspeaker, a flat and substantially rectangular frame of nonmagnetic material having a coil wound thereon, a pair of spring blades supported at one end and connected at the other end to opposite portions of said frame, said spring blades being arranged flatwise in a plane substantially at right angles to the plane of said frame, whereby said spring blades permit bodily vibratory movement of the frame in its own plane and prevent lateral movement thereof, a field magnet structure operatively associated with said coil, and a diaphragm connected to said frame.

9. In a loudspeaker,a flat and substantially rectangular frame having a coil wound thereon, a field magnet having a pair of aligned airgaps in which two paralled sides of said frame vibrate so that the corresponding sides of the coil cut the magnetic flux across said airgaps, a diaphragm connected to one of said sides of the frame, lugs projecting from the other two sides of said frame, a pair of spring blades attached at one end to said lugs and arranged fiatwise in a plane substantially at right angles to the plane of said frame, and a support to which the other ends of said spring blades are connected.

10. In a loudspeaker, a flat and substantially rectangular frame having a coil wound thereon, a field magnet having a pair of aligned airgaps in which two parallel sides of said frame vibrate so that the correspond ing sides of the coil cut the magnetic flux across said airgaps, a diaphragm connected to one of said sides of the frame, lugs projecting from the other two sides of said frame, a pair of spring blades attached at one end to said lugs and arranged flatwise in a plane substantially at right angles to the plane of said frame, a support to which the other ends of said spring blades are connected, and means for adjusting the normal tension of said spring blades, said adjusting means including a turnable member on said support.

ADOLPH A. THOMAS. 

